The present invention relates to a communication system and, more particularly, to a tuning device for an equalization system applicable to data signalling.
In the data signalling art, tuning systems, or adaptive control systems, associated with modulated signals heretofore proposed include a decision reference system. A decision reference is more advantageously applicable to a system with relatively small intersymbol interference than an absolute reference and makes the mutual interference insignificant. In that case, a relatively short tuning and a small hardware scale suffice. An output signal of an equalizer which is adapted to equalize a signal received over a circuit may be developed in various eye patterns depending upon the phase and amplitude distortions which the received signal have suffers due to circuit. The situation concerning the decision of symbols of received signals is most delicate when the signals are significantly distorted in phase through the circuit. The fluctuation in the phase of a carrier is indefinite so that in the case of symbol decision which uses two levels, for example, the phase sometimes vary over the two discrete decision ranges. Then, those symbols which accidentally lie in an unexpected one of the decision ranges commands the equalizer to erroneously correct equalization parameters.
One approach for solving the above problem is disclosed in Japanese Unexamined Patent Publication (Kokai or Kaku), No. 57-111135, for example. In accordance with the disclosed tuning system, an insensitive area, for symbol decision is set up in a phase plane and, when a received signal lies in the insensitive area, correction on the equalizer which is based on an error of the received signal relative to a reference signal is not performed.
As well known in the art, in a demodulator system which includes an equalizer, a equalizer section of the demodulator is trained before the reception of desired data signals over a circuit. During a training sequence, the symbol usually changes from a two-phase symbol to a multi-phase symbol as the sequence proceeds. As prescribed by CCITT Recommendation V.27ter, for example, two-phase symbols may be used to segment 4 and, then, four- or eight-phase symbols in segment 5. Such is effective to efficiently converge various parameters of the equalization system.
In the prior art system which sets up an insensitive area as described, the insensitive area is unchangeable, or fixed. Because the training sequence is of the two-phase type, the decision of an insensitive area can be made with ease. However, if convergence is not attained within the conditioning pattern of the equalizer (segment 4 in CCITT V27ter), it will not be attained thereafter. That is, even after the equalization system has begun to converge, the error correcting function is limited to in turn limit the attainable degree of equalization.